Wireless communication systems operating in the same geographical area or adjacent spectral bands interfere with one another and require large guard bands to minimize such interference which results in an inefficient usage of scarce spectrum resources. The Downlink (DL)/Uplink (UL) interference problem that typically occurs between APs (access points) of uncoordinated cells is illustrated for example in FIG. 1.
If both APs use FDD based duplexes (i.e. pure FDD or half duplex FDD) the interferer will be sufficiently separated in frequency from the wanted signal such that receiver filtering and out-of-band emission requirement specification are realizable and obtainable such that interference is dealt with sufficiently.
However, if one or both APs is using a TDD based duplex in one of its bands (i.e. pure TDD, DL/UL oriented hybrid, dual band hybrid or band switching FDD) then for uncoordinated APs, one AP will be receiving at the same time as the other AP is transmitting in the same or adjacent bands.
In the worse case scenario the APs of different operators are co-located. The minimum coupling loss can be increased from 30 dB to 45 dB by careful positioning of receive antennas, however the receive filter and transmitter out-of-band emission requirements will be very high in order to not generate harmful interference. Therefore, the co-location of different operators APs is not a viable or practical option.
Typically, spectral masks are used in wireless communication systems to allow systems to operate next to each other or in vicinity of one another. The spectrum masks have been defined in IST-2003-507581 WINNER, “D2.5 v1.0 Duplex arrangements for future broadband radio interfaces”, which is incorporated herein by reference, based on the noise floor and ACPR (adjacent channel power ratio) requirements. TX spectrum masks for AP and MT in NB and WB modes are illustrated for example in FIG. 2.
The estimates for guard band requirements based on the transmitter noise floor requirements are presented both in relative and absolute terms in Table 1 shown in FIG. 4. It can be seen from Table 1 that such a strict guard band definition results in wide guard bands, in the order of one-to-three times the signal bandwidth. The values presented in Table 1 prevent any coexistence of the duplex schemes without excessive guard bands and impose extremely demanding transmitter noise floor requirements.
Coupling losses between interfering transmitter and victim receiver can be calculated based on cable losses, antenna gains and free space path loss at the carrier frequency of e.g. 5 GHz withLc[dB]=32.4+20*log(f[MHz])+20*log(d[km])+CRx[dB]+CTx[dB]−ARx[dB]−ATx[dB]  [equation 1]where, Lc is the coupling loss, f is the carrier frequency, d is the physical separation, CRx and CTx are the cable losses, and ARx and ATx are the antenna gains at the transmitter and receiver, respectively.
In FIG. 3 it was assumed that the spectrum values decrease 19 dB/bandwidth after the first bandwidth until it reaches the noise floor. This is shown by the steeper part of the spectrum mask illustrated in FIG. 2. Thus, if the coupling loss could be increased by 19 dB, then the guard band requirements could be reduced by more than 100 MHz for a 100 MHz signal bandwidth and the required guard band would be in a reasonable range. The only part of the coupling loss Lc [dB] of equation 1 that can be influenced is the propagation part {20*log(d[km])}, which assumes free space propagation.
Currently there is a discussion in the United Kingdom about the auctioning of the 2.5 GHz band and the FDD/TDD bands separation. The situation is such that guard bands are not defined and the discussion assumes that the operators working on neighboring bands will coordinate their sites to avoid the interference problem.
If one operator builds its network before such cooperation, the operator builds and plans it without considering such interference. If then another operator builds its network, for example two years later, then the other operators' network might deteriorate the performance of the first operators' network. As history has shown us, this may lead to unacceptable situations and such a “radio war” led to the founding of the Federal Communication Commission in the United States. Therefore, the operators will need a technical solution to help them coordinating their sites.
Accordingly, what is needed is a way for two operators to build and operate its respective wireless communication network without causing harmful interference or receiving harmful interference from the other regardless of the time the respective wireless communication networks are built.